Fiber Bragg gratings (FBG) and other refractive index structures in optical fiber have found widespread applications in such technology areas as optical telecommunications, optical fiber sensors, and optical fiber lasers (cf. e.g. Raman Kashyap, “Fiber Bragg gratings”, Academic Press, 1999 (ISBN: 0-12-400560-8), referred to as [Kashyap] in the following and Andreas Othonos, Kyriacos Kalli, “Fiber Bragg gratings”, Artech House, 1999 (ISBN: 0-89006-344-3), referred to as [Othonos, Kalli] in the following). The fabrication of FBG's involves illuminating the fiber with a spatially modulated pattern of actinic radiation (typically UV-laser light). This is typically established by interference between laser beams, e.g. by transmitting the actinic radiation through a phase mask. The process requires the fiber to be photo-sensitive such that its refractive index is modified under the influence of actinic radiation. Photo-sensitivity is often achieved by doping the fiber in or around the core with a sufficiently high concentration of germanium or other suitable dopant materials (cf. [Kashyap], [Othonos, Kalli]). However, often doping by itself does not provide sufficient photo-sensitivity. In such cases the photo-sensitivity can be enhanced considerably by in-diffusion of molecular hydrogen or deuterium (cf. U.S. Pat. No. 5,235,659, “Method of making an article comprising an optical waveguide”). This process, which is most often referred to as loading, is often carried out by subjecting the fiber to high pressures (P >>1 bar) for a sufficiently long time. The process can be accelerated by using elevated temperature.
This way the partial pressure of molecular hydrogen or deuterium in the doped section of the fiber can be brought to a level high enough that Ge and molecular hydrogen or deuterium react under the influence of e.g. UV-light to bring about the desired level of refractive index change.
At the same time, micro-structured fiber (or crystal fiber) has gained acceptance over recent years as a new and revolutionizing technology that opens new degrees of freedom in tailoring the guidance properties of optical fiber (cf. e.g. U.S. Pat. No. 6,539,155, “Microstructured optical fibres”). Micro-structured fiber is characterized by the presence of elongated cavities (or air holes) in the fiber. The geometry of the cavities (e.g. the cross-sectional size and distribution, and mutual centre-centre distances) defines the guidance properties of the fiber, and in principle no Ge or other dopant is needed for guidance. Thus a single material (e.g. pure silica) fiber can be constructed, and properties such as cut-off wavelength and dispersion can be designed to meet specifications that cannot be obtained by standard fiber fabrication methods. Another advantage of micro-structured fiber lies in the ability to produce air-clad optical fiber wherein the core is surrounded by an inner and an outer cladding and the inner and outer cladding are separated by a structure of air holes (cf. e.g. U.S. Pat. No. 5,907,652, “Article comprising an air-clad optical fiber”). The ring of air holes surrounding the inner cladding results in a high refractive index step or a high numerical aperture (typically around 0.6). This provides for an efficient coupling of pump light to the inner cladding. At the same time the pump light is detached from the polymer coating surrounding the outer cladding. The inner cladding can thus accommodate very high power levels of multiple spatial mode pump light—an essential building block for the fabrication of high power fiber lasers.
Specifically the combination of crystal fiber and fiber Bragg gratings opens new application possibilities. One example is the field of fiber optical sensors where gas or fluids can be guided through the hollow sections of the fiber and thus influence the reflective properties of FBG's. Another example is the field of high power fiber lasers, where air-clad optical fiber will allow for coupling of very high levels of pump power while FBG's act as cavity defining reflectors.
The process of imprinting FBG's in micro-structured fiber has however shown to pose a special problem, namely that it has been difficult to perform the UV-imprinting.